INCREDIBLE INSECTS: A Celebration of Insect Biology
Monarch Butterflies
Monarch Migration
In the autumn each year, monarch butterflies from the northern United States and Canada begin their southward migration. Populations west of the Rocky Mountains migrate to places along the California coast, where they overwinter. Pacific Grove, on the Monterey peninsula, is a famous monarch overwintering site. Those east of the Rockies migrate primarily to a small region in the mountains of central Mexico, where they aggregate in the millions. The southward migration passes through Durham in October each yea, although it is not always noticeable because the monarchs fly quite high. In the spring, the overwintering monarchs mate, and the females begin a northward migration. They lay a few eggs on their way north, and you can find them on milkweed plants in Durham in May every year. The butterflies mostly migrate to more northern regions where milkweed plants are abundant and where they reproduce in large numbers. The adult butterflies die after a few weeks, and when the next generation of caterpillars metamorphoses to adults, they begin the southward autumn migration again. The overwintering site in Mexico is small, only a few square miles. How monarchs from across the entire Midwest and northeast manage to find it is still a great puzzle.
Monarch Decline
With a range that stretches from southern Canada to southern Mexico, the monarch butterfly is familiar to most residents of North America. However, pressures imposed by humans are pushing this iconic animal to the brink of endangerment. Over the past twenty years, monarch populations have declined by as much as 80% at overwintering sites. The causes of the monarch’s struggles are still being studied, but one of the biggest threats to their continued existence is undoubtedly the widespread destruction of their host plants. Adult monarchs lay their eggs on milkweed (Asclepias spp.), which is later consumed by emerging larvae. Milkweed is the only source of food for monarch larvae, so the loss of these plants has an immediate and often catastrophic effect populations of adult butterflies.
Monarch Conservation
Although the continued existence of the monarch is now threatened, non-profit organizations such as Monarch Watch are making an effort to help the species rebound through organized planting of milkweed, population monitoring, and other activities. The animals shown above have been tagged with labels Monarch Watch provides to educational institutions and citizen scientists. Volunteers capture and tag adult monarchs with a uniquely numbered label. Details such as the date and location of tagging are then entered into an online database. When the animal arrives at its overwintering area, scientists are able to collect important data about its migratory route, and other life history characteristics.
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